The preparation of water-soluble, acidic polymers is generally carried out either by the modification of neutral polymers to introduce acidic groups onto the polymer chain or by solution polymerization of acidic monomers. Modifications of neutral polymers, for example, hydrolysis of polyacrylamide or carboxymethylation of cellulose, are generally difficult and costly processes, and often lead to the degradation of the polymer with resulting lowering of the molecular weights of the modified polymers. The solution phase polymerization of acidic monomers, such as acrylic acid, is a efficient and economical reaction, but generally results in polymers of only moderately high molecular weights, which are not effective thickeners or floculating agents.
The solution phase polymerization of acidic monomers is generally carried out by adding a free radical initiating catalyst to a solution of an appropriate concentration of the monomer. Since the acidic polymers are generally used in aqueous solutions, e.g. as floculating agents, the solvent in these polymerizations is usually water. The water solvent also helps to control the exothermic nature of the reaction. The initiating catalysts are chemicals or combinations of chemicals which generate free radicals during the course of the reaction. When normal precautions are taken, such as exclusion of air and the use of low catalyst to monomer ratios in these reactions, the resulting polymers have only moderately high molecular weights, i.e., up to several hundred thousand.
When acidic polymers of very high molecular weights are desired for applications such as thickening agents and floculating agents, the usual practice is to modify very high molecular weight natural or synthetic polymers so as to form acidic groups along the already formed polymer chain. An example of such a practice is in the preparation of very high molecular weight polyacrylic acid, where acrylamide is polymerized to a molecular weight of several million and then subsequently hydrolyzed with acid or base to convert the amide groups to carboxylic acid groups. This practice is inefficient and costly and is only used because it is not possible to polymerize acrylic acid directly to such high molecular weights.